The invention relates to a process and a device for separation of an acid gas fraction comprising carbon dioxide and/or hydrogen sulfide contained in a gas mixture, e.g. a sour gas, that comprises at least one lighter component, such as methane.
The technological background is illustrated by U.S. Pat. Nos. 4,152,129, 5,983,663 and 5,927,103.
The separation of carbon dioxide that is contained in a gas mixture is an operation that is intended either to obtain a gas that contains a reduced content of carbon dioxide, for example in the case of a natural gas, with a view to obtaining an adequate gross calorific value or to obtaining a carbon dioxide-concentrated gas. Such a gas can be used, for example, to carry out an assisted recovery operation of petroleum. In addition, the wish to limit carbon dioxide emissions into the atmosphere leads to a growing advantage for operations for reinjecting carbon dioxide underground. In this case, it is generally preferable to inject underground a carbon dioxide-concentrated gas either to avoid losing enrichable fractions or to reduce the cost of the compression and injection operation.
There exist different processes for treatment of a gas such as a natural gas or a refinery gas to separate the carbon dioxide. These processes are generally based on an operation of absorption by a chemical or physical solvent. These processes are limited, however, to the treatment of a gas containing relatively small contents of carbon dioxide, for example not exceeding 20 mol %.
In the case of a gas mixture that contains high carbon dioxide contents, there exist very few suitable solutions.
The Ryan-Holmes process, which is most used in this case and which is based on a series of stages of fractionation by distillation, is a costly process. This process that operates under cold conditions typically uses four distillation columns, operating in the presence of an additive, and consists of a hydrocarbon fraction that is recycled so as to avoid any risk of crystallization of the carbon dioxide.
In the same way, it may be necessary to separate from a gas mixture, such as a natural gas, another acid gas such as hydrogen sulfide (H2S). There exist processes that make it possible to separate such a contaminant that is toxic and corrosive, for example by washing the gas mixture with an amine. These processes become very costly, however, as soon as the gas mixture comprises more than 10 to 15% of H2S.
The process according to the invention aims at separating an acid gas such as CO2 and/or H2S in a more economical manner by reducing the amount of equipment needed as well as the energy consumption.
It was discovered, and this is a first object of this invention, that it is possible to carry out such an operation in a simple and economical manner by operating the refrigeration and the rectification of the gas mixture simultaneously in an indirect heat exchanger.
More specifically, the invention relates to a process for separation of an acid gas fraction that comprises carbon dioxide and/or hydrogen sulfide that is contained in a mixture that comprises at least one lighter gas, characterized in that it comprises the following stages:
(a) gas mixture (1) is precooled at least once during a heat exchange operation (E2),
(b) the gas mixture that is obtained from stage (a) is cooled and rectified simultaneously in an approximately vertical heat exchange zone (ER) by generating a downward liquid reflux,
(c) a gaseous fraction (8) that is low in acid gases and high in light gas is collected at the top of the vertical exchange zone, and
(d) an acid gas-enriched liquid fraction (4) is collected at the bottom of the exchange zone.
Such a cooling and rectification exchanger is, for example, a plate exchanger. The gas mixture that is to be separated circulates vertically in the upward direction in the exchanger while being cooled. The circulation speed of the gas mixture is kept at a low enough value for the liquid fraction that is generated by cooling to be able to descend again in countercurrent to the gas mixture. In going down, this liquid fraction is reheated and enriched with carbon dioxide while the gas mixture exits at the top with a substantially reduced carbon dioxide content.
By extracting heat by indirect exchange, more reflux is created and thus a better acid gas (CO2 for example) /light gas (hydrocarbon for example) fractionation is obtained.
The gaseous fraction that is low in acid gases, collected during stage (c), can comprise methane and/or nitrogen and/or hydrogen.
According to a first variant of the invention, it was also discovered that the refrigeration that is necessary to the gas mixture can be ensured at least in part by being expanded at least once and by evaporating in the exchanger at least in part the acid gas-rich liquid fraction collected at the base of the exchanger in which the fractionation of the mixture is carried out.
According to a characteristic of this invention, the expanded liquid fraction is separated in a separation chamber B2, and an acid gas-rich gaseous top fraction that is recycled in stage (a) is recovered. It is preferable to carry out at least in part a first expansion of the liquid fraction and to separate a first expansion effluent in a first separation chamber in such a way as to recover at the top a first gaseous fraction that is enriched with lighter components than the acid gases recycled in stage (a) and a first acid gas-enriched liquid fraction. This first liquid fraction can be expanded at least in part, and the second expansion effluent can be separated into a second gaseous top fraction that is enriched with lighter components than the acid gases, recycled in precooling stage (a), and into a second acid gas-enriched liquid fraction that is used as a coolant in the heat exchanger. These first and second gaseous fractions that are enriched with lighter components than the acid gases and at two different pressure levels can be recompressed. The fact of operating in at least two stages makes it possible to reduce the mechanical energy of compression that is necessary for recycling the separated gaseous fractions in stage (a).
According to another embodiment, the acid gas-enriched liquid fraction that is collected at the end of stage (d) can be expanded successively to at least two pressure levels, whereby liquid fraction (4a) that is obtained at the end of a first expansion stage is brought into contact in a zone (C1) for countercurrent contact with a vapor reflux (16) that is obtained from the evaporation of a portion of liquid fraction (5a) that is collected at the bottom of said contact zone (C1).
According to a second variant of the invention, the refrigeration that is required during stage (b) can be ensured at least in part by expansion (TD) of gaseous fraction (8) that is low in acid gas and high in light gas that is collected during stage (c).
According to a third variant of the invention, the refrigeration that is required during stage (b) can be ensured at least in part by external refrigeration means, for example an external refrigeration cycle that operates with propane.
According to another characteristic of the process, it is also possible to operate the contact in countercurrent that is carried out in exchanger ER between the acid gas-rich gaseous fraction and the liquid reflux in the presence of solvent.
This offers various advantages. First of all, it thus is possible to inhibit the formation of solid crystals of carbon dioxide and thus to be able to drop to a lower temperature, for example xe2x88x9250xc2x0 C., at the top of exchanger ER. By operating at a lower temperature, the amount of acid gas(es) entrained with the lighter gas, for example methane and/or nitrogen with which this or these acid gas(es) are mixed, is reduced.
The addition of a solvent may also make it possible to improve the quality of the separation, primarily if this solvent has a selective nature compared to the acid gas(es) to be separated.
This solvent is preferably a polar solvent with a low enough viscosity to be able to be used at relatively low temperature, such as, for example, methanol, dimethyltetraethylene glycol or propylene carbonate, pure or mixed. Any physical solvent that can be used to carry out a deacidification operation can be considered.
It is also possible to use a light hydrocarbon, if such a hydrocarbon is produced on site, to keep any solvent from being fed in.
The presence of solvent also makes it possible to improve the separation between the CO2 and/or the H2S and the hydrocarbons by preventing azeotrope formation, in particular between C02 and ethane, as well as between H2S and ethane or propane. The amount of solvent can vary in large proportions according to the application and the degree of separation desired, and the molar ratio of the amount of solvent to the amount of extracted acid gases can be between, for example, 0.5 and 10.
Under these conditions, an optionally cooled liquid stream that comprises the solvent can be sent to the top of the exchange zone in which stage (b) is carried out by collecting, at the bottom of said exchange zone (ER), acid gas-enriched liquid fraction (4) and at least a portion of the solvent that is sent to the top.
The acid gas-enriched liquid fraction, collected at the bottom of exchange zone ER, can be at least partially expanded, separated and reheated in exchange zone (E2) in which the operation of stage (a) is carried out by generating an acid gas-enriched gaseous fraction (23) and a liquid fraction (20) that is recycled in stage (b).
The operating conditions of the process are generally as follows:
precooling stage (a), up to a temperature that is close to the dew point temperature of the gas, between, for example, +10xc2x0 C. and xe2x88x9220xc2x0 C.,
stage (b) for ensuring indirect contact in the heat exchanger.
Pressure: 10 to 70 bar and preferably 30 to 60 bar
(1 bar=105 Pa) Temperature:
xe2x88x9210 to xe2x88x9250xc2x0 C. without solvent
xe2x88x9210 to xe2x88x9280xc2x0 C. with solvent
Generally, the temperatures that are used to separate a CO2-rich acid gas are lower than those for separating an H2S-rich acid gas. The gas that is thus treated can thus be a natural gas or an industrial gas such as a synthesis gas or a refinery gas, whereby it can contain at least one acid gas.
The process applies to various compositions of the mixture of acid gases to be separated.
The acid gases to be separated can comprise carbon dioxide and/or hydrogen sulfide separately or mixed. They can also comprise other acid contaminants, such as mercaptans, COS, or CS2.
The lightest gases from which they are separated can comprise methane and/or nitrogen separately or mixed. They can also comprise other light gases such as hydrogen, as well as hydrocarbons such as ethane, propane or even heavier hydrocarbons, some of which can be recovered with the acid gases at the end of the separation that is carried out by the process.
The process preferably can apply for a CO2 content that is at least equal to 20 mol % or for an H2S content that is at least equal to 10 mol %.
Each of the fractions that are separated by the process can undergo additional treatments according to various methods of one skilled in the art.
The acid gas fraction that is obtained can be fractionated, for example by distillation, to separate each of the acid gases that it contains as well as the hydrocarbons that can be found with this fraction.
The at least partially deacidified gaseous fraction can undergo an additional treatment, for example by washing by solvent.
The countercurrent contact between the upward gaseous fraction and the downward liquid fraction that is carried out in the exchanger denoted ER in the diagrams of FIGS. 1 to 4 can be operated at pressures of between, for example, 10 and 70 bar.
The temperature at the top of this exchanger can drop to about xe2x88x9250xc2x0 C. in the absence of solvent and to about xe2x88x9280xc2x0 C. in the presence of solvent.
The necessary refrigeration can be ensured by evaporation of a liquid fraction that comprises acid gases that are separated by the process. It can also be ensured by expansion of the gaseous fraction that is low in acid gases or by other means that are known to one skilled in the art such as, for example, an external refrigeration cycle that operates with propane.
At least a portion of the acid fraction can be pumped to be reinjected under pressure via at least one injection well underground (a reservoir, for example) with a view to carrying out an assisted recovery of petroleum.
In the presence of solvent, it is also possible to fractionate a gas mixture that contains water. In this case, the solvent is to be at least partially water-miscible to prevent the formation of ice or hydrate crystals. This solvent can be, for example, methanol. In this case, the water is recovered with the solvent and can be separated from methanol, for example by bringing this solvent phase into contact with at least one fraction of the feedstock gas.
The invention also relates to a device for separating an acid gas fraction that is contained in a mixture that comprises at least one lighter gas, characterized in that it comprises:
at least one indirect cooling means (E2) of the mixture that has a first inlet connected to a mixture feed (1) and an outlet;
an indirect heat exchanger ER that is approximately vertical and that comprises at its base a separating tank B1 that is connected to the outlet of cooling means E2, whereby said exchanger comprises means (MF) for circulating a coolant and means (MC) for circulating a hot fluid that is connected to separating tank B1;
means (8) for recovery at the top of heat exchanger ER of a gaseous fluid that has been cooled, high in light gas and low in acid gases, connected to a second inlet of indirect cooling means E2;
means (4) for recovery of a liquid fluid that is high in acid gases and low in light gases at the bottom of separating tank B1.
Means (4) for recovery of the acid gas-enriched fluid can comprise at least one fluid expansion means V1 connected to means for circulating the coolant in heat exchanger ER, whereby said circulation means of the evaporated coolant are connected to a third inlet of indirect cooling means E2 to deliver an acid gasenriched gas.
According to a first variant of the device, the expansion means can be coupled to a first separating tank B2 that comprises an upper outlet for a gas that contains acid gases connected to a fourth inlet of indirect cooling means, and a lower outlet for an acid gas-enriched liquid, whereby said lower outlet comprises a second expansion means V3 coupled to a second separating tank B3 that comprises an upper outlet for a gas that contains acid gases connected to a fifth inlet of the indirect cooling means, whereby said indirect cooling means has a fourth outlet and a fifth outlet for gas that contains acid gases corresponding to the fourth and fifth inlets connected to gas mixture feed (1) via a compressor (K1); whereby said second separating tank (B3) has a lower outlet connected to means for circulating the coolant in exchanger ER.
According to a second variant, the means for recovery of acid gas-enriched fluid comprise expansion means V2 coupled to a column C1 that comprises packing elements, whereby the column has an upper outlet for a gas that contains acid gas connected to the gas mixture feed via indirect cooling means E2 and a compressor K1, and a lower outlet 5a for an acid gas-concentrated liquid, whereby a second expansion means V3 is connected to the lower outlet for the liquid and to means (MF) for circulating the coolant in the upper part of exchanger ER at a temperature T2, whereby said exchanger comprises in its lower part another means (15, 16) for circulating coolant at a temperature T1 greater than T2, connected to lower outlet (5a) of the liquid and to the lower part of said column C1.
According to a third variant, the means for recovery of light gas-enriched gaseous fluid comprise an expansion turbine TD of gaseous fluid connected to means (MF) for circulating coolant in exchanger ER, whereby said refrigeration means are connected to the second inlet of cooling means E2, said means (8) for circulating light gas-enriched hot fluid, for example hydrocarbons that comprise an inlet for a solvent (20) into exchanger ER, means for recovery of acid gas-enriched liquid fluid comprising an expansion means V2 coupled to a contact column (C1), whereby said column has an upper outlet for acid gas-enriched gas that passes through cooling means E2 of the mixture, and a lower outlet (21) for acid gas- and solvent-enriched liquid connected to cooling means (E2) then to a separating tank B2, whereby said separating tank has an upper outlet for acid gas connected to the lower part of contact column C1, and a lower outlet for liquid containing the solvent connected to the top of the exchanger with means (MC) for circulating hot fluid via a pump P1.